1. Field of the Invention
The present invention relates to an electronic camera and, more particularly, to a technique for reducing noise contained in a still image.
2. Description of the Related Art
In this kind of electronic camera, a writing operation is performed in such a manner that an analog signal output from an image sensor at the time of still image photography is AD-converted and the AD-converted digital signal is written to a random access memory (RAM).
Japanese Patent Application Publication No. 10-13799 discloses an image reader designed so that the frequency of the basic operating clock signal is reduced except during photography and accompanying processing in order to reduce wasted electric power.
Ordinarily, electric power consumed for writing a large amount of data in the form of a digital signal to a RAM is large. A noise source is therefore produced at the time of writing and noise is mixed in the analog signal output from the image sensor, resulting in degradation in image quality. This problem is serious when the level of the analog signal output from the image sensor is low (that is, when the subject is dark).
Also, a direct memory access (DMA) transfer method is ordinarily used to write an AD-converted signal to a RAM with efficiency. If noise is mixed in the analog signal during DMA transfer, several vertical streaks appear in the entire image and image degradation become easily recognizable, as shown in FIG. 12. In this case, the influence of noise is increased.
The invention disclosed in Japanese Patent Application Publication No. 10-13799 has the effect of reducing electric power by reducing the frequency of the basic operating clock signal except during photography and accompanying processing, but has a problem that the image quality is affected by noise from a power supply section during photography as in the above-described case since the camera is operating on the high-frequency basic operating clock signal during photography.
In recent years, the development of image sensors having higher resolution has been pursued. It is a common practice to increase the frequency of the basic operating clock signal in order to increase the speed of digital processing for signal readout from a high-resolution image sensor. In such a case, however, a captured image is affected by noise due to an increase in the frequency of the basic operating clock signal.